Almost all of the presidents can be characterized as Christian, at least by upbringing, though some were unaffiliated with any specific religious body. Some are thought to have been deists, or irreligious. No president thus far has been openly an atheist or an adherent of any non-Christian religion.

Most presidents have been formal members of a particular church or religious body, and a specific affiliation can be assigned to every president from Garfield on. For many earlier presidents, however, formal church membership was forestalled until they left office; and in several cases a president never joined any church. Conversely, though every president from Washington to John Quincy Adams can be definitely assigned membership in an Anglican or Unitarian body, the significance of these affiliations is often downplayed as unrepresentative of their true beliefs.

The pattern of religious adherence has changed dramatically over the course of United States history, so that the pattern of presidential affiliations is quite unrepresentative of modern membership numbers. For example, Episcopalians are extraordinarily well represented among the presidents, compared to a current membership of about 2% of the population; this is partly because the Episcopal Church had been a part of the Church of England before the American Revolution and was the state religion in some states (such as New York and Virginia) and the church was much larger in the past with a decline occurring only in recent years.[7] The first seven presidents listed as Episcopalians were all from Virginia. Unitarians are also overrepresented, reflecting the importance of those colonial churches. Conversely, Baptists are underrepresented, a reflection of their quite recent expansion in numbers; there has been only one Catholic president, although they are currently the largest single denomination, and there have been no Lutheran, Pentecostal, or Latter Day Saint presidents.

While many presidents did not formally join a church until quite late in life, there is a genre of tales of deathbed conversions. Biographers usually doubt these, though the baptism of Polk is well documented.

The inner beliefs of the presidents are much more difficult to establish than church membership. While some presidents have been relatively voluble about religion, many have been reticent to the point of complete obscurity. Researchers have tried to draw conclusions from patterns of churchgoing or religious references in political speeches. When explicit statements are absent, it is difficult to assess whether the presidents in question were irreligious, were unorthodox in their beliefs, or simply believed that religion was not a matter for public revelation.

On the other hand, there are several presidents who considered themselves aligned with a particular church, but who withheld from formal affiliation for a time. Buchanan, for instance, held himself allied with the Presbyterian church, but refrained from joining it until he left office.

Some presidents changed their beliefs and affiliation at some point in their lives; synthesis of statements and membership from different periods can be misleading.

Deism was a religious philosophy in common currency in colonial times, and some Founding Fathers (most notably Thomas Paine, who was an explicit proponent of it, and Benjamin Franklin, who spoke of it in his Autobiography) are identified more or less with this system. No president identified himself as a deist, but George Washington, Thomas Jefferson, James Madison, James Monroe, and John Tyler are often identified as having some degree of deistic beliefs.[8] Washington in particular maintained a lifelong pattern of church membership and attendance, and there is conflicting testimony from those who knew him.

Four presidents are affiliated with Unitarian churches, and the fifth (Jefferson) was an exponent of ideas now commonly associated with Unitarianism. Unitarians fall outside of Trinitarian Christianity, and the question arises as to the degree to which the presidents themselves held Christian precepts. The information is generally available in the statements of the presidents themselves; for example, John Quincy Adams left detailed statements of his beliefs. William Howard Taft, a Unitarian, is noted to have said in a letter to a friend, "I am interested in the spread of Christian civilization, but to go into a dogmatic discussion of creed I will not do whether I am defeated or not. . . . If the American electorate is so narrow as not to elect a Unitarian, well and good. I can stand it."[9]

Two presidents were Quakers (Herbert Hoover and Richard Nixon) and information about their religion is harder to come by. Quakerism is, by its nature, not circumscribed by doctrines, but even so it is hard to determine whether either Hoover or Nixon had much adherence even to Quaker practice. For instance, it is common among Quakers to refuse to swear oaths; however, recordings show that Nixon did swear the oath of office in the conventional manner in all cases, and while the matter is clouded for Hoover, there is newspaper and circumstantial evidence that he did likewise.

The only other president with any association with a definitely non-Trinitarian body is Eisenhower, whose parents moved from the River Brethren to the antecedents of the Jehovah's Witnesses. Eisenhower himself was baptized in the Presbyterian church shortly after assuming the presidency, the only president thus far to undergo such a rite while in office; and his attendance at West Point was in sharp opposition to the pacifist tenets of the groups to which his parents belonged.

There are some presidents for which there is little evidence as to the importance of religion in their lives. For example, almost no evidence exists for Monroe's personal religious beliefs, though this may be the result of the destruction of most of his personal correspondence, in which religious sentiments may have been recorded. As with claims of deism, these identifications are not without controversy. No president has declared himself to be irreligious, agnostic, or atheist.

St. John's Episcopal Church, just across Lafayette Square, north of the White House, and built in 1815–1816, is the church nearest to the White House, and its services have been attended at least once by nearly every president since James Madison (1809–1817). Another Episcopal church, Washington National Cathedral, chartered by Congress in 1893, has been the scene of many funeral and memorial services of presidents and other dignitaries, as well as the site of interfaith presidential prayer services after their inaugurations.

Presidential proclamations, from the earliest days, have often been laden with religious if not explicitly Christian language. In at least two cases, presidents saw fit to issue denials that they were atheists. At the same time, this was tempered, especially in early years, by a strong commitment to disestablishment. Several presidents especially stand out as exponents of this. Consideration of this has become increasingly contentious as topics such as civil rights and human sexuality have increasingly put churches at odds with each other and with the government.

Presidential biographers have often been brought to consider the issue of presidential religion. In the case of certain key figures (particularly Washington, Jefferson, and Lincoln), they have devoted considerable attention to the subject.

Some researchers have produced general surveys of presidential religion. A recent example is The Faiths of the Founding Fathers by David L. Holmes (New York, Oxford University Press USA, 2006), which examines the views of some early presidents as well as other political figures of the period. The Adherents.com website maintains a list of presidential affiliations, with subpages for each president.[8] Most of these subpages refer to a site by one Peter Roberts, which has links and some more detailed information on the religion of the presidents, vice presidents, and founding fathers.[10]

The Adamses were originally members of the state-supported Congregational churches in New England. By 1800, most Congregationalist churches in Boston had Unitarian preachers teaching the strict unity of God, the subordinate nature of Christ, and salvation by character.[14][15][16] Adams himself preferred Unitarian preachers, but he was opposed to Joseph Priestley's sympathies with the French Revolution, and would attend other churches if the only nearby Congregational/Unitarian one was composed of followers of Priestley.[17]

"Like many others of his time (he died just one year after the founding of institutional Unitarianism in America), Jefferson was a Unitarian in theology, though not in church membership. He never joined a Unitarian congregation: there were none near his home in Virginia during his lifetime. He regularly attended Joseph Priestley's Pennsylvania church when he was nearby, and said that Priestley's theology was his own, and there is no doubt Priestley should be identified as Unitarian. Jefferson remained a member of the Episcopal congregation near his home, but removed himself from those available to become godparents, because he was not sufficiently in agreement with the Trinitarian theology. His work, the Jefferson Bible, was Unitarian in theology..."

In a letter to Benjamin Rush prefacing his "Syllabus of an Estimate of the Merit of the Doctrines of Jesus", Jefferson wrote:

"In some of the delightful conversations with you, in the evenings of 1798–99, and which served as an anodyne to the afflictions of the crisis through which our country was then laboring, the Christian religion was sometimes our topic; and I then promised you, that one day or other, I would give you my views of it. They are the result of a life of inquiry & reflection, and very different from that anti-Christian system imputed to me by those who know nothing of my opinions. To the corruptions of Christianity I am indeed opposed; but not to the genuine precepts of Jesus himself. I am a Christian, in the only sense he wished any one to be; sincerely attached to his doctrines, in preference to all others; ascribing to himself every human excellence; & believing he never claimed any other."[22]

Although Madison tried to keep a low profile in regards to religion, he seemed to hold religious opinions, like many of his contemporaries, that were closer to deism or Unitarianism in theology than conventional Christianity. He was raised in the Church of England and attended Episcopal services, despite his personal disputes with the theology.[24]

Monroe was raised in a family that belonged to the Church of England when it was the state church in Virginia, and as an adult attended Episcopal churches.[25]

"When it comes to Monroe's ...thoughts on religion", Bliss Isely comments in his The Presidents: Men of Faith, "less is known than that of any other President." Monroe burned much of his correspondence with his wife, and no letters survive in which he discusses his religious beliefs; nor did his friends, family or associates write about his beliefs. Letters that do survive, such as ones written on the occasion of the death of his son, contain no discussion of religion.[25]

Some authors conclude that Monroe's writings show evidence of "deistic tendencies".[25]

He was a founding member of the First Unitarian Church of Washington (D.C.).[27] However he regularly attended Presbyterian and Episcopal services as well.[27]

Towards the end of his life, he wrote, "I reverence God as my creator. As creator of the world. I reverence him with holy fear. I venerate Jesus Christ as my redeemer; and, as far as I can understand, the redeemer of the world. But this belief is dark and dubious."[27]

Polk came from a Presbyterian upbringing but was not baptized as a child, due to a dispute with the local Presbyterian minister in rural North Carolina. Polk's father and grandfather were Deists, and the minister refused to baptize James unless his father affirmed Christianity, which he would not do.[39][40] Polk had a conversion experience at a Methodist camp meeting when he was thirty-eight, and thereafter considered himself Methodist. Nevertheless he continued to attend Presbyterian services with his wife, though he went to the local Methodist chapel when she was ill or out of town. On his deathbed, he summoned the Rev. John B. McFerrin, who had converted him years before, to baptize him.[38]

Rev. Dr. Phineas D. Gurley, pastor of the New York Avenue Presbyterian church in Washington D.C., which Lincoln attended with his wife when he attended any church, never claimed a conversion. According to D. James Kennedy in his booklet, "What They Believed: The Faith of Washington, Jefferson, and Lincoln", "Dr. Gurley said that Lincoln had wanted to make a public profession of his faith on Easter Sunday morning. But then came Ford's Theater." (p. 59, Published by Coral Ridge Ministries, 2003) Though this is possible, we have no way of verifying the truth of the report. The chief evidence against it is that Dr. Gurley, so far as we know, never mentioned it publicly. The determination to join, if accurate, would have been extremely newsworthy. It would have been reasonable for Dr. Gurley to have mentioned it at the funeral in the White House, in which he delivered the sermon which has been preserved.[46] The only evidence we have is an affidavit signed more than sixty years later by Mrs. Sidney I. Lauck, then a very old woman. In her affidavit signed under oath in Essex County, New Jersey, February 15, 1928, she said, "After Mr. Lincoln's death, Dr. Gurley told me that Mr. Lincoln had made all the necessary arrangements with him and the Session of the New York Avenue Presbyterian Church to be received into the membership of the said church, by confession of his faith in Christ, on the Easter Sunday following the Friday night when Mr. Lincoln was assassinated." Mrs. Lauck was, she said, about thirty years of age at the time of the assassination.

John Remsburg, President of the American Secular Union, argued against claims of Lincoln's conversion in his book Six Historic Americans (1906). He cites several of Lincoln's close associates:

The man who stood nearest to President Lincoln at Washington – nearer than any clergyman or newspaper correspondent – was his private secretary, Col. John G. Nicolay. In a letter dated May 27, 1865, Colonel Nicolay says: "Mr. Lincoln did not, to my knowledge, in any way change his religious ideas, opinions, or beliefs from the time he left Springfield to the day of his death."

After his assassination Mrs. Lincoln said: "Mr. Lincoln had no hope and no faith in the usual acceptance of these words." His lifelong friend and executor, Judge David Davis, affirmed the same: "He had no faith in the Christian sense of the term." His biographer, Colonel Lamon, intimately acquainted with him in Illinois, and with him during all the years that he lived in Washington, says: "Never in all that time did he let fall from his lips or his pen an expression which remotely implied the slightest faith in Jesus as the son of God and the Savior of men."[47]

He accompanied his wife Eliza McCardle Johnson to Methodist services sometimes, belonged to no church himself, and sometimes attended Catholic services—remarking favorably that there was no reserved seating.[49]

Grant was never baptized into any church, though he accompanied his wife Julia Grant to Methodist services. Many sources list his religious affiliation as Methodist based on a Methodist minister's account of a deathbed conversion. He did leave a note for his wife in which he hoped to meet her again in a better world.

In his 1875 State of the Union address, during conflicts over Catholic parochial schooling, Grant called for a constitutional amendment that would require all states to establish free public schools while "forbidding the teaching in said schools of religious, atheistic, or pagan tenets; and prohibiting the granting of any school funds or school taxes... for the benefit... of any religious sect or denomination."[50] The proposed Blaine Amendment to the Constitution followed.

Hayes came from a Presbyterian family, but attended Methodist schools as a youth.[51]

Many sources list him as Methodist; in general, however, it is agreed that he held himself to be a Christian, but of no specific church.[52]

In his diary entry for May 17, 1890, he states: "Writing a few words for Mohonk Negro Conference, I find myself using the word Christian. I am not a subscriber to any creed. I belong to no church. But in a sense, satisfactory to myself and believed by me to be important, I try to be a Christian, or rather I want to be a Christian and to help do Christian work."[53]

Hayes' wife, Lucy, was a Methodist, a temperance advocate, and deeply opposed to slavery; He generally attended church with her.[52]

James Rusling, a McKinley supporter, related a story that McKinley had addressed a church delegation and had stated that one of the objectives of the Spanish-American War was "to educate the Filipinos, and uplift and civilize and Christianize them".[62] Recent historians have judged this account unreliable, especially in light of implausible[vague] statements Rusling made about Lincoln's religion.[63][64]

McKinley is the only president to include exclusively Christian language in his Thanksgiving Day proclamation.[65]

Roosevelt always stated that he was Dutch Reformed; however, he attended Episcopal churches where there was no Reformed church nearby. (His second wife Edith was Episcopalian from birth.)[66] As there was no Dutch Reformed church in Oyster Bay, New York, he attended Christ Church Oyster Bay when in residence there, and it was in that church that his funeral was held.[66]

His mother was Presbyterian and as a child he attended Presbyterian churches with her.[67]

Before becoming president, Taft was offered the presidency of Yale University, at that time affiliated with the Congregationalist Church; Taft turned the post down, saying, "I do not believe in the divinity of Christ."[69]

Taft's beliefs were the subject of some controversy, and in 1908 he found it necessary to refute a rumor that he was an atheist.[4]

As Quakers customarily do not swear oaths, it was expected that Hoover would affirm the oath of office, and most sources state that he did so.[75][76] However, a Washington Post article dated February 27, 1929, stated that he planned to swear, rather than affirm, the oath.[77]

Eisenhower's religious upbringing is the subject of some controversy, due to the conversion of his parents to the "Bible Student" movement, the forerunner of the Jehovah's Witnesses, in the late 1890s. Originally, the family belonged to the River Brethren, a Mennonite sect.[81] According to the Eisenhower Presidential Library, there is no evidence that Eisenhower participated in either the Bible Student group or the Jehovah Witnesses, and there are records that show he attended Sunday school at a River Brethren church.[81]

Until he became president, Eisenhower had no formal church affiliation, a circumstance he attributed to the frequent moves demanded of an Army officer. He was baptized, confirmed, and became a communicant in the Presbyterian church in a single ceremony February 1, 1953, just 12 days after his first inauguration, the only president to undergo any of these rites while in office.[81]

Eisenhower was instrumental in the addition of the words "under God" to the Pledge of Allegiance in 1954 (an act highly promoted by the Knights of Columbus), and the 1956 adoption of "In God We Trust" as the motto of the USA, and its 1957 introduction on paper currency. He composed a prayer for his first inauguration, began his Cabinet meetings with silent prayer, and met frequently with a wide range of religious leaders while in office.[81]

His presidential library includes an inter-denominational chapel in which he, his wife Mamie, and his firstborn son (who died in childhood) are buried.

Reagan's father was Catholic,[89] but Reagan was raised in his mother's Disciples of Christ denomination and was baptized there on September 21, 1922.[90] Nancy and Ronald Reagan were married in the Disciples of Christ "Little Brown Church" in Studio City, California on March 4, 1952. Beginning in 1963 Reagan generally attended Presbyterian church services at Bel Air Presbyterian Church, Bel-Air, California. During his presidency he rarely attended church services, due to the inconvenience to others in the congregation.[91] He became an official member of Bel Air Presbyterian after leaving the Presidency. Reagan stated that he considered himself a "born-again Christian".[88]

^Rusling, James (22 January 1903). "Interview with President William McKinley". The Christian Advocate: 17. Reprinted in Daniel Schirmer and Stephen Rosskamm Shalom, ed. (1987). The Philippines Reader. Boston: South End Press. pp. 22–23.